In WO 00/19065 there is a description of a gas turbine blade and a method for producing a gas turbine blade. In order to form such a cast gas turbine blade through which coolant flows and which has passage openings, at least one of which is induced by casting production, in such a way that they can be produced with little scrap and largely avoiding casting defects, it is proposed that one or more of the passage openings induced by the casting process is or are passed through by a rivet-like closure pin, the foot of which is secured on the opposite side of an end wall.
To accomplish a high efficiency of the gas turbine, a working gas temperature that is as high as possible is required. The blades of the gas turbine, which are subjected to considerable loading as a result of high or changing temperatures, pressures and centrifugal forces, are metallic hollow bodies, the hollow spaces of which are flowed through by coolant. The coolant flow proceeds from the root of the gas turbine blade, by which the latter is fastened to a rotor, through an inner hollow space with meandering air ducting chambers to outlet openings in the airfoil region or in the pointed region of the gas turbine blade and forms a cooling film on the outer wall of the gas turbine blade.
To minimize the proportion of unused working gas flowing past the edge region of the gas turbine blades, the rotor with the inserted gas turbine blades is arranged inside the gas turbine in such a way that the envelope of the tips of the gas turbine blades extends at the smallest possible distance from the inner circumference of a peripheral, static guide ring. In this respect it is necessary to ensure not only a close fit but also that the gas turbine blade is not damaged and the radially outer tip region is adequately cooled. In this tip region, the outer wall of the gas turbine blade on the tip side is therefore surrounded by projecting edge pieces, the tip squealers. Cooling of this tip squealer region takes place by means of a coolant flow which flows from the inner hollow spaces through passage openings in the end wall on the tip squealer side into the tip squealer region and is led out again there through openings in the tip squealers, as disclosed in U.S. Pat. No. 4,761,116.
A gas turbine blade with internal cooling and passage openings for cooling the tip squealer region constructed on a single level is known from EP 0 340 149 B1. The production of such a gas turbine blade imposes increased requirements on the construction of the casting molds and in particular the core of the casting molds. To ensure consistent cooling of the tip squealer region, the thickness of the end wall on the tip squealer side between the hollow space and the tip squealer region must be produced with close tolerances. The core part of the system of hollow spaces is rigidly connected to the core part of the tip squealer region and in this way the two are kept at a fixed, constant distance from each other during the casting process. This connection is established by securing pins, which are anchored in both core parts. The securing pins are subjected to great loading during the casting process, as a result of which they easily break. As a result, the securing pins are to be formed with a relatively great diameter.
However, this great diameter has the effect of producing large passage openings in the end wall on the tip squealer side of the cast gas turbine blade between the hollow space and the tip squealer region. Consequently, the leakage of the coolant flowing through the passage openings is very great. One possible way of reducing the leakage of the coolant is to weld the large passage openings produced by the securing pins closed or to weld a cover plate onto them. This has the disadvantage that it may have the effect of inducing cracks, which increase during the operation of the gas turbine blade. The high temperatures during the welding may also cause recrystallization processes, which weaken the material at the locations concerned, in particular in the case of monocrystalline and directionally solidified gas turbine blades. The great loading caused by the centrifugal force when the gas turbine blades are rotating leads more frequently to the partial or complete detachment of the cover plate.
U.S. Pat. No. 3,761,201 describes a method for closing the casting-induced openings in a gas turbine blade. A closure pin which has a higher coefficient of thermal expansion than the surrounding material is inserted into the casting-induced openings in an exactly fitting manner. Diffusion welding is achieved by subsequent heating.
In the book “Wissensspeicher Lasertechnik” [compendium of laser technology] by Witlof Brunner and Klaus Junge, VEB Fachbuchverlag Leipzig 1989, laser machining and welding is described on pages 291 to 305. The construction of a laser material-machining installation is shown for example by FIG. 4.4 on page 291. A laser beam is directed in a rigidly arranged laser beam guide onto a workpiece, which is arranged on a displaceable working bench. The guidance of the working being carried out on the workpiece takes place by the displacement of the workpiece by means of the second working bench. The laser welding installations described on the subsequent pages also provide guidance for the working by means of the movement of the workpiece. Welding with lasers is discussed on page 297.